No One Really Knows What a Shamrock Is

The three-leaf clover is what everyone wears, but what species is it?

This St. Patrick’s Day, shamrocks will be everywhere: on clothing, shot glasses, beer mugs, funny hats and other sometimes questionable fashion accessories. It’s easy to think of those three bright green leaves as inviolably Irish, an icon of the Emerald Isle since the beginning of time. According to Irish folklore, the shamrock is so entirely Irish it won’t even grow on foreign soil. And in America, only the three-leaved image of the shamrock persists, having been associated with Irish immigrant communities for more than 100 years—it’s just as important on St. Patrick’s Day as wearing green clothing and drinking emerald-hued libations. The catch, however, is that shamrocks, at least as a term of scientific nomenclature, don’t really exist.

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The “shamrock” is a mythical plant, a symbol, something that exists as an idea, shape and color rather than a scientific species. Its relationship to the plant world is a bit like the association between cartoon hearts we draw and the anatomical ones inside our bodies. The word "shamrock" first appears in plays and poetry in the 1500s, but the first person to link it to a recognizable plant was the English herbalist John Gerard, who in 1596 wrote that common meadow trefoil, also known as clover, was "called in Irish Shamrockes." Botanists have been trying to match the idea of the shamrock with a particular species for centuries, so far without unanimous success. Although the plant is assumed to be a type of clover—the term “shamrock” comes from the Gaelic seamrog, or "little clover"—the clover genus (Trifolium) includes hundreds of species. Other herbs, such as wood sorrel, have also been labeled and sold as “shamrock” over the years. The confusion stems in part from the time of year when St. Patrick’s Day approaches on the calendar: In Ireland, the holiday comes along in spring, when plants are at their most nascent stages and many species are just sprouting leaves. When fully grown, white clovers bloom white flowers and red clovers bloom reddish flowers (naturally), but most laypeople won’t be able to tell the difference when pinning just the baby clover leaves on a jacket.

Of course, attempts to pinpoint the shamrock’s species aren’t exactly of earth-shaking significance. No wars have been fought over their true nature, no fortunes ruined, no reputations destroyed. At most, it’s caused 19th-century botanists writing in natural history journals to get a little flushed in the face.

In 1830, James Ebenezer Bicheno, a London botanist and colonial official stationed in Ireland, claimed that the true shamrock was Oxalis acetosella, or wood sorrel. He based his claim in part on selections from Irish literature and traveller reports that described the Irish eating shamrocks in times of war and disaster, arguing the “sharp” taste reported in those descriptions matched wood sorrel better than clover. Bicheno also claimed, falsely, that clover wasn’t native to Ireland, and that it was a relatively recent addition to the countryside, while wood sorrel would have been more plentiful in days of yore. In 1878, English botanists James Britten and Robert Holland addressed the "vexed question" of the true shamrock by saying the Trifolium minus (yellow clover) was the species most often sold as shamrock in Covent Garden on St. Patrick’s Day, although they noted that Medicago lupulina (black medick) occasionally took its place, and was more often sold in Dublin.

About ten years later, Nathaniel Colgan, a young police clerk and amateur botanist in Dublin, decided to make matters more scientific. Writing in an 1892 edition of The Irish Naturalist, Colgan noted “the species of the Shamrock had never been seriously studied by any competent botanist … perhaps because any attempt to go into it exhaustively may have been checked at the outset by the thought that the Irishman was content to wear, as the national badge, any well-marked trifoliate leaf. Such a thought, however, could only have entered the mind of an alien. Every Irishman … well knows that the Irish peasant displays great care in the selection of his Shamrock. There is for him one true Shamrock and only one.”

Seeking to find a scientific answer to the question of the “one true Shamrock,” Colgan asked correspondents in 11 Irish counties to collect, around the time of St. Patrick's Day, samples of shamrocks they considered to be the real deal. After potting them and allowing them to flower, Colgan discovered that eight were Trifolium minus (yellow clover) and five Trifolium repens (white clover). He repeated the study the following year, after contacting clergymen in parishes around the country to send more samples. This time, out of a total of 35 specimens, 19 were white clover, 12 yellow clover, 2 red clover and 2 black medick. The results varied by county, with many parts of Ireland evenly split along yellow and white, while the counties of Cork and Dublin favored the black medick. (Colgan’s initial experiment had avoided Dublin and its environs, where he felt "the corrosive rationalism of cities" would blunt "the fine instinct which guides the Irish Celt in the discrimination of the real Shamrock.")

Almost a century later, in 1988, E. Charles Nelson, then horticultural taxonomist in Ireland's National Botanic Gardens, decided to repeat the study to see if anything had changed. Nelson made an appeal in the national press asking Irish people to send examples of plants they considered the “real shamrock” to the Botanic Gardens. This time, he found that yellow clover accounted for 46 percent of the 243 samples, followed by white clover at 35 percent, black medick at 7 percent, wood sorrel at 5 percent and red clover at 4 percent. The results were very similar to Colgan’s study, showing that Irish ideas of the “real” shamrock had held steady. The experiments “also demonstrated that there is no single, uniquely Irish species that can be equated with shamrock,” as Nelson wrote.

According to Dublin-based writer and tour guide Mary Mulvihill, it was 20th-century international commerce that forced the need to settle on a single species, at least for export. “When the Department of Agriculture had to nominate an ‘official’ one for commercial licenses to companies that export shamrock, it chose the most popular species, yellow clover (T. dubium),” she writes. Today, T. dubium is the species sold most often as shamrock by commercial growers in Ireland, and it’s the most likely seed to be in packets labeled “true” shamrock, which are mostly sold to gullible tourists, according to Nelson.

But what makes the search for the true shamrock so loaded with meaning? It goes back to the day, and the man, most closely related to the symbol. Legend has it that St. Patrick, patron saint of Ireland, used the three-leaf clover to explain the concept of the Holy Trinity (Father, Son and Holy Ghost) in the fourth century A.D. while converting the Irish to Christianity. (St. Patrick, by the way, is the one who was supposed to have driven all the snakes out of Ireland, although scholars today say the serpents were a metaphor for paganism.) But the story of St. Patrick and the shamrock, as we know it, is just that: There’s no mention of the shamrock in the saint’s writings, and the first written reference to the idea of St. Patrick using the plant to explain the Trinity is in the early 18th century, more than a thousand years after his supposed lessons. That reference appears in the first book ever published about Irish plants, written by Caleb Threlkeld, a British minister and doctor. In his Synopsis Stirpium Hibernicarum, Threkeld writes of white clover:

"This plant is worn by the people in their hats on the 17th day of March yearly, which is called St Patrick’s day. It being the current tradition that by this 3-leafed grass [Patrick] emblematically set forth the mystery to them of the Holy Trinity.”

He added, judgmentally: “However that be, when they wet their Seamar-oge [shamrock], they often commit Excess in Liquor … generally leading to debauchery.”

These days, few believe St. Patrick actually used the shamrock. “If he did use a three-leaved plant to explain the Trinity, he probably wouldn’t have chosen something as tiny as the shamrock,” says Mulvihill. “He probably would have used bog bean or something with bigger leaves—something you could see at the back of the hall.”

But aside from its connection to St. Patrick’s Day, the shamrock is firmly rooted in Irish history. At some point in the Middle Ages, shamrocks started showing up in the floral emblems of Britain and Ireland, appearing alongside English roses, Scottish thistles and Welsh leeks, according Nelson, who is also author of Shamrock: Botany and History of an Irish Myth. The earliest reference to the wearing of shamrocks is in 1681, and by the 1720s the plants were worn on hats. In the beginning of the 1800s, they started showing up as popular decorative motif carved into churches, splashed across fashion and jewelry, and festooning books and postcards. By the 1820s almost anything meant to have an Irish connection had a shamrock on it, Nelson says. Over time, wearing the shamrock would alternate between being a charged nationalist symbol to a more innocent display of Irish pride.

In the end, the species of the “true shamrock” may not matter. Attempts to translate the cultural world into the scientific one can be fraught (witness the debate over what to call the symbol of this year’s Chinese New Year). But if the shamrock provides a cultural touchstone, a way to transmit the idea of Irishness all across the world, that’s likely what’s most important. And besides, yellow clover, wood sorrel and black medick all probably taste the same drowned in whiskey.

This article originally referred to Charles Nelson as the onetime director of the Irish Botanical Gardens. He was actually a horticultural taxonomist at the National Botanic Gardens, which the text now indicates.

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